1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a transmission/reflection combination type LCD capable of displaying an image in a transmission mode and in a reflection mode.
2. Description of the Background Art
In recent years, liquid crystal display devices, having desirable characteristics such as a small thickness and a small power consumption, have been widely used in various applications, including OA equipment such as word processors and personal computers, PDAs (personal digital assistance) such as electronic organizers, and camcorders with liquid crystal monitors.
These liquid crystal display devices are generally classified into those of reflection type and those of transmission type. A liquid crystal display device is not self-luminous as is a CRT (cathode ray tube) or an EL (electroluminescence) device. A transmission type liquid crystal display device displays an image by using light from an illuminator (so-called “backlight”) arranged on the rear side of the liquid crystal display panel, and a reflection type liquid crystal display device displays an image by using ambient light.
A transmission type liquid crystal display device, which displays an image by using light from the backlight, is capable of producing a bright display with a high contrast ratio without being substantially influenced by the brightness of the environment, but consumes a lot of power due to the backlight. Moreover, a transmission type liquid crystal display device has a poor visibility under very bright environments (e.g., when used outdoor under a clear sky).
On the other hand, a reflection type liquid crystal display device, which does not have a backlight, consumes little power, but the brightness and the contrast ratio thereof are substantially influenced by the conditions under which it is used, e.g., the brightness of the environment. Particularly, the visibility lowers significantly under dark environments.
In order to overcome these problems, transmission/reflection combination type LCDs, which are capable of operating both in a reflection mode and in a transmission mode, have been proposed in the art.
A transmission/reflection combination type LCD includes, in each picture element region, a reflection picture element electrode that reflects ambient light and a transmission picture element electrode that transmits light from the backlight, and is capable of displaying an image selectively in a transmission mode or in a reflection mode depending on the environment under which it is used (e.g., the brightness of the environment). A transmission/reflection combination type LCD is also capable of displaying an image by using both modes at the same time. Thus, a transmission/reflection combination type LCD has a low power consumption, which is an advantage of a reflection type liquid crystal display device, and is capable of producing a bright display with a high contrast ratio without being substantially influenced by the brightness of the environment, which is an advantage of a transmission type liquid crystal display device. Moreover, the decrease in visibility when used under very bright environments (e.g., when used outdoor under a clear sky), which is a drawback of a transmission type liquid crystal display device, is suppressed.
As described above, a transmission/reflection combination type LCD displays an image by using light from the backlight in a transmission region and by using ambient light in a reflection region. As a result, the number of times light passes through the liquid crystal layer in the transmission region differs from that in the reflection region. Therefore, the thickness of the liquid crystal layer in the transmission region is set to be larger than that in the reflection region so that the optical path length for display light passing through the transmission region is matched with that for display light passing through the reflection region (see, for example, Japanese Laid-Open Patent Publication No. 2000-305110). Typically, the thickness of the liquid crystal layer in the transmission region is set to be about twice that in the reflection region.
However, for a liquid crystal display device in which the thickness of the liquid crystal layer in the reflection region is larger than that in the transmission region, i.e., a liquid crystal display device having a so-called “multi-gap structure”, the optimal structure for forming multiple gaps has not yet been discovered. For example, Japanese Laid-Open Patent Publication No. 2000-305110 discloses a method in which a step is provided on an active matrix substrate, which is one of a pair of substrates opposing each other via a liquid crystal layer therebetween provided on the rear side of the device. The present inventors have found that this structure has a problem of a decrease in the brightness.